U.S. Pat. No. 5,925,472 (Hu et al., Xerox Corporation) discloses inter alia an organic electroluminescent device comprising in the following sequence a substrate, an anode, a hole injection layer, a hole transport layer, an electron transport layer, a so-called “electron injection layer”, and a cathode in contact with the electron injection layer. The so-called electron injection layer may be comprised of a metal oxinoid compound, e.g. tris-(8-hydroxyquinolinate)-aluminum, or bis-(8-quinolinethiolato)-zinc. The cathodes exemplified are of MgAg. Cathodes of lithium alloyed with other high work function metals such as aluminium or indium are disclosed but not exemplified. The use of aluminium cathodes is neither disclosed nor suggested. It is not apparent that the disclosed layers are true electron transport layers which should be very thin, in the case of organic materials not more than 10 nm and preferably less than 7 nm. The range of thicknesses disclosed by Hu et al. for their so-called electron transport layer is 5-80 nm. In the examples the deposited films are of thickness 80 nm and 30 nm, and these films are identified as electron transport layers rather than electron injection layers. Devices at a constant current of 25 ma/m2 gave initial light outputs of 350-450 cd/m2 and reduction in light output to 50% of initial intensity over periods of 150-210 hours of continuous operation, so that the effective life of the disclosed devices is short.
Hung et al., “Recent progress of molecular organic electroluminescent materials and devices”, Materials Science and Engineering, R 39 (2002), 143-222 disclose that bilayer cathodes for OLEDs based e.g. on a thin (0.1-1.0 nm) LiF layer between an aluminium cathode and an aluminium quinolate electron transport layer exhibit significantly improved I-V characteristics and EL efficiencies compared to a MgAg cathode, see also U.S. Pat. No. 5,776,622 (Hung et al., Kodak). In a comparison of current/voltage characteristics of three OLEDs using Al, MgAg and Lif/Al as cathodes, the OLED with the Al cathode required higher drive voltages than that with MgAg, whereas that using Al/LiF required lower drive voltages. Hung et al explain that in OLEDs, the majority carriers are holes owing to their higher mobility and smaller injection barrier. Therefore, lowering the barrier height to electron injection is especially important as it leads to a better balance of electron and hole currents and results in a dramatic increase in luminance at a fixed bias voltage. The replacement of LiF with CsF or alkaline earth fluorides is also discussed.
U.S. Pat. No. 6,558,817 (Ueda et al., Minolta) provides a general disclosure of the use of alkali metal and alkaline earth salts and complexes to form electron injection layers in electroluminescent devices. A list of 100 representative compounds said to be useful as electron injectors is given, amongst which is lithium quinolate is mentioned. Acetylacetonates are said to be preferred, but the present applicants have found that the use of these materials gives rise to sensitivity to moisture, processing difficulties, poor device lifetimes and relatively large drift voltages. Useful cathode materials are said to include aluminium, indium, magnesium, calcium, titanium, yttrium, lithium, gadolinium, ytterbium, ruthenium, manganese and alloys thereof. The preparation of lithium quinolates with suitable purities and properties is not described, lithium quinolate is not used in any of the examples, there is no disclosure or suggestion of the combination of an aluminium cathode and a lithium quinolate or substituted quinolate electron injection layer, and there is no discussion of the problem of finding a combination of materials with better overall properties than Al/LiF. The exemplified OLEDs are claimed to exhibit stable luminescence over a long service life, but this is supported by measurements of light output over a period of only five hours, times to reduction in light output of 50% of initial intensity are not quoted, and in truth the lifetimes of the exemplified devices are short.
U.S. Pat. No. 6,885,149 (Parthasarathy et al., Princeton University) discloses that during fabrication of an OLED, an organic electron injection layer may be doped with a metal either by depositing an organic electron injection layer on an ultra-thin layer of lithium or by depositing an ultra-thin layer of lithium on an organic electron injection layer, the organic material being e.g. 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP or bathocuproine), see also US 2004/0085016 (Parthasarathy et al.). Use of a metal doped electron injection layer is also known in the art wherein, for example, the organic component of the electron injection layer being e.g. is the compound shown below:
